1. Field of the Invention
The present invention relates to a method for processing fluorescent material. More particularly, the present invention relates to a method of processing fluorescent material for reuse, the fluorescent material recovered at the developing step during formation of the fluorescent screen of a color image receiving tube.
2. Discussion of the Background Art
A conventional process used to form the fluorescent screen of a color image receiving tube is shown in FIG. 1. First the fluorescent material is prepared. Then it is combined with ammonium dichromate, polyvinyl alcohol, and a surfactant in water to form a slurry. Third, the slurry is injected into the inner face of the panel, which is the faceplate of the color image receiving tube, and then coated thereon with a rotary coater at a high rotation rate, forming a smooth coating. In the fourth step, the fluorescent coating formed on the panel is dried, a shadow mask is mounted to a inner face of the panel, and the coating is exposed, forming cross-linkages between polyvinyl alcohol and sodium dichromate. The fifth step comprises developing the fluorescent coating in warm water to remove any unexposed fluorescent material, thus forming a fluorescent screen. These steps are carried out repeatedly so that green, blue and red fluorescent materials are coated in sequence. As a result, the fluorescent screen of a stripe form or a dot form is obtained.
The amount of unexposed fluorescent material removed from the panel during the development step can be up to 70 to 80% of total fluorescent material initially prepared. Conventionally, these materials are discarded as is, or recovered for reuse after being washed with warm water. It is wasteful to discard these fluorescent materials, particularly red fluorescent material, which contains expensive rare earth elements, and, additionally, the heavy metals and sulfur components of the materials add to environmental pollution. Therefore, the fluorescent materials which come off the panel in the developing step should be recovered.
In a conventional method for recovering the fluorescent material washed from the panel in the developing step, the material is recovered by centrifugation. The recovered fluorescent material contains not only polyvinyl alcohol and surfactant from the slurry, but also various impurities and oil components involved in the process for forming fluorescent screen. To reuse the recovered fluorescent material, the slurry ingredients, impurities, and oil components must be removed.
FIG. 2 shows a conventional method for processing recovered fluorescent material. The recovered fluorescent material is first dispersed in water to dissolve particle cohesion. Then, the dispersed fluorescent material is cleaned with a heated aqueous alkali solution, held quiescent to drain the supernatant, and washed with water, ridding the material of the slurry ingredients, impurities, and oil components. The fluorescent material is dehydrated, dried, and passed through mesh for screen-sizing.
A drawback of this conventional method for processing recovered fluorescent material is that the steps of cleaning the fluorescent material with a heated aqueous alkali solution, draining the supernatant, and washing with water, must be repeated at least 2 to 3 times to adequately remove the impurities from the recovered fluorescent material. This process is therefore tedious and time-consuming.
As stated above, the fluorescent screen of a color image receiving tube is formed by coating and developing fluorescent materials of green, blue, and red color in sequence. The screen is not complete until the fluorescent materials of all three primary colors are coated and developed. When a fluorescent coating of blue is formed after patterning a green fluorescent coating, a slurry of the blue fluorescent material is coated on the inner face of the panel on which the green fluorescent coating is already patterned. When developing the blue fluorescent coating, green fluorescent material that is loosely attached to the panel is collected with the remnants of the blue fluorescent material. Similarly, subsequent application of red fluorescent material to the panel results in small amounts of both blue and green material in the red fluorescent material recovered during developing.
The conventional processing step in which the recovered fluorescent material is cleaned with a heated aqueous alkali solution cannot separate fluorescent materials of different colors. Therefore, if the fluorescent screen of a color image receiving tube is formed with recovered fluorescent materials incorporating more than one color, the desired luminescence may not occur. For example, FIG. 3 shows the fluorescent material of green color incorporated with the recovered fluorescent material of blue color. The two fluorescent materials will luminesce together, even when only blue fluorescent material is required to luminesce. Also, the fluorescent material of green color or the fluorescent material of blue color incorporated with the recovered fluorescent material of red color will luminesce together with the fluorescent material of red color, even when only a red luminescence is required. Thus, incorporation of one fluorescent material with another detrimentally affects the luminescent color purity and the picture quality of a fluorescent screen.
The fluorescent material may be attached to a pigment by a pigment-attaching agent and this material may also be recovered for reuse. A method for processing such pigment-attached fluorescent material is disclosed in Japanese Patent Publication No. Sho 59-7747. This method comprises the steps of recovering the pigment-attached fluorescent material, adding the recovered fluorescent material to an aqueous alkali solution to corrode the pigment-attaching gelatin, and attaching a suitable amount of new pigment to the resultant fluorescent material, making it reusable.
This method for processing pigment-attached fluorescent material is much less successful when latex, not gelatin, is used as the pigment-attaching agent because the aqueous alkali solution cannot corrode the latex. Another method, however, exists in which the pigment attached to fluorescent material with latex is removed by baking the recovered fluorescent material at 450.degree. C. This method incinerates the latex, leaving the ashes within the fluorescent material. These ashes remain even after repeated cleaning attempts and use of the recovered fluorescent material in a fluorescent screen can cause staining and low luminous efficiency.
In another conventional process for forming the fluorescent screen of a color image receiving tube, a slurry of fluorescent material is prepared by dispersing fluorescent material in water and adding polyvinyl alcohol in combination with sodium dichromate and surfactants. The sodium dichromate is a photosensitive agent. This slurry is coated on an inner face of a panel, dried, and exposed to ultra violet rays. The ultra violet rays cause the dichromate in the slurry to react with the polyvinyl alcohol. The polyvinyl alcohol is changed into .alpha.-ketonic acid and .beta.-hydroxide, which are insoluble in water.
The photo-crosslinking reaction occurring in the slurry is as follows. The dichromic acid of the photosensitive agent changes into hexavalent chromic acid upon reception of the UV rays. The hexavalent chromic acid links with polyvinyl alcohol to form an ester, which subsequently forms a ketone group, and the ketone group becomes an unsaturated ketone due to an intramolecular dehydration reaction. The unsaturated ketone changes into a chromic acid ester, which is hydrolyzed to form .alpha.-keto-1,2-glycol. The .beta.-keto-1,2-glycol is cleaved into .alpha.-ketonic acid linked with trivalent chrome and a .beta.-hydroxy acid. As a result, the slurry becomes insoluble in water.
There are disadvantages, however, in the use of recovered fluorescent material processed by this method. In fluorescent screens formed from a slurry of this material, the coating in areas receiving insufficient light or exhibiting low cohesiveness may easily deteriorate. In attempting to solve this problem, the exposure amount may be increased, but then the fluorescent coating becomes to thick and interferes with the part on which it is being formed. This results in a false, mixed color on the fluorescent screen. In addition, because the photo-crosslinking reaction is affected by water, the coating must be sufficiently dry before exposure can take place. This increases the time and energy required by the process.